The plasma CVD process means a process that produces a plasma of a specific material containing highly active radicals and causes the radicals to deposit a film on the surface of a substrate. Plasma CVD apparatus means an apparatus that is employed in carrying out the plasma CVD process.
The conventional plasma CVD apparatus comprises a plasma CVD chamber comprising a vacuum vessel provided with a material gas inlet opening and a discharge opening and an electromagnetic wave generating device for supplying energy for producing a plasma of a material gas introduced into the plasma CVD chamber.
Incidentally, the plasma CVD process is based on the high activity of the radicals, and the conditions for the plasma CVD process, such as the density of radicals and the temperature of a work, are properly selected so that a desired deposited film may be formed. In this respect, the efficient production of radicals is essential in the plasma CVD process.
In the past, a high-frequency electromagnetic wave of 13.56 MHz had been used for the plasma CVD process. Recently, it has been found that the application of a microwave of a frequency with the order of 2.45 GHz in the plasma CVD process enables efficient production of a high-density plasma and simultaneously, heating the work. Thus, a plasma CVD process using a microwave has become the object of attention and several microwave plasma CVD apparatus have been proposed. For instance, there have been proposed various plasma CVD processes using a microwave (hereinafter referred to as "MW-PCVD process") and apparatus for carrying out the same for forming a deposited film of amorphous silicon (hereinafter referred to as "A-Si"), polycrystal silicon (hereinafter referred to as "p-Si"), SiO.sub.2 or SiN for use in preparing, for example, semiconductor devices, photoconductive members for electrophotography, image input sensors, image pickup devices, photovoltaic devices, other electronic devices and optical devices.
These proposed conventional MW-PCVD apparatus are classified roughly into two types.
MW-PCVD apparatus of one of the two types are disclosed in Japanese Patent Publication Nos. 58-49295 and 59-43991 and Japanese Utility Model Publication No. 62-36240. In a MW-PCVD apparatus of this type (hereinafter referred to as "type 1 MW-PCVD apparatus"), a gas pipe is inserted through or placed in contact with a rectangular or coaxial waveguide to produce a plasma.
A MW-PCVD apparatus of the other type is disclosed in Japanese Patent Laid-open (Kokai) No. 57-133636. In this MW-PCVD apparatus (hereinafter referred to as "type 2 MW-PCVD apparatus"), electron cyclotron resonance (ECR) is established within a cavity resonator and a plasma is drawn out by a divergent magnetic field.
FIG. 5 shows a typical type 1 MW-PCVD apparatus disclosed in Japanese Utility Model Publication No. 62-36240. This type 1 MW-PCVD apparatus comprises a vacuum system, an exhaust system and a microwave generating system.
Referring to FIG. 5, the vacuum system comprises a reactor 107, and a microwave transmissive tube, such as a quartz tube, or a window having an inside diameter on the order of 40 mm and connected to the reactor 107 by a gas introducing pipe 107a. The quartz tube (or the window) is connected to a first gas introducing pipe and is arranged perpendicularly to a microwave waveguide. A second gas introducing pipe is connected to the reactor 107. A gas, such as silane gas, supplied into the reactor 107 is exhausted through the exhaust system (107b and 108). A gas, such as oxygen gas or nitrogen gas, introduced through the first, gas introducing pipe into the reactor 107 is converted into a plasma by microwave discharge. During the microwave discharge caused by microwave energy, microwave impedance can be matched with a sliding short-circuit plate, i.e., a plunger 105. Radicals of the plasma thus produced react with the silane gas supplied through the second gas introducing pipe, whereby a silicon dioxide film or a silicon nitride film is formed over the surface of a substrate 111.
FIG. 6 shows a typical type 2 MW-PCVD apparatus disclosed in Japanese Patent Laid-open (Kokai) No. 57-133636. The systems and configuration of the type 2 MW-PCVD apparatus are the same as those of the foregoing type 1 MW-PCVD apparatus except that the type 2 MW-PCVD apparatus employs an electromagnet 13. In the type 2 MW-PCVD apparatus, the vacuum system comprises a cylindrical plasma producing vessel 1 and a deposition vessel 2 connected to the plasma producing vessel 1. A microwave introducing window 3 is attached hermetically to the plasma producing vessel 1. A first gas introducing pipe 6 and a microwave waveguide 4 are connected to the plasma producing vessel 1. The plasma producing vessel 1 is water-cooled by means of a water-cooling pipe 5. The apparatus shown in FIG. 6 is provided with an electromagnet 13 disposed coaxially with the plasma producing vessel 1. The direction of the magnetic lines of force of the electromagnet 13 is the same as the direction of advancement of a microwave. Electrons move for a magnetron motion under the combined force of a magnetic field formed by the electromagnet 13 and an electric field formed by the microwave. Therefore, the plasma producing vessel 1 is formed in a cavity resonator of a TE.sub.11t mode (t=a natural number). A second gas introducing pipe and the exhaust system are connected to the deposition vessel 2. Gases staying within the deposition vessel 2 are exhausted by the exhaust system.
The typical type 2 MW-PCVD apparatus shown in FIG. 6 converts a gas (hydrogen gas) through the first gas introducing pipe 6 into the plasma producing vessel 1 into a plasma by discharge caused by the microwave energy. When the magnetic flux density of the magnetic field is 875 gauss, the reflected wave of the microwave energy is almost zero. In this apparatus, the end plate 16 of the cavity resonator having the construction of a choke is moved under a vacuum according to the type of the gas, the pressure of the gas and the microwave power applied to the cavity resonator so that the cavity resonator meets required conditions. Electrons of the hydrogen plasma move for an electron cyclotron motion in the direction of the magnetic lines of force, and the radicals of the plasma react with the gas (silane gas) introduced through the second gas introducing pipe into the deposition vessel 2 to form an a-Si film over the surface of a substrate 11.
Both the type 1 MW-PCVD apparatus and the type 2 MW-PCVD apparatus, however, have the following problems to be solved.
That is, the type 1 MW-PCVD apparatus has disadvantages: (i) the interior of the reactor must be maintained at a pressure of 1 torr or higher to maintain stable discharge: (ii) deactivation of the radicals during travel through the gas introducing pipe reduces film forming deposition rate: and (iii) a portion of the quartz tube at the junction of the quartz tube and the waveguide is caused to sputter by the concentrated intensity of the electric field and sputtered particles are mixed in the deposited film to deteriorate the electrical properties of the deposited film when the input microwave power is increased to increase the film forming rate.
Although there is no problem attributable to the deactivation of radicals and the sputtering of the quartz tube in the type 2 MW-PCVD apparatus, the type 2 MW-PCVD apparatus has the following problems.
That is, (iv) since film formation is performed under a pressure on the order of 10.sup.-4 torr, where the mean free path of the radicals is about 1 m, a-Si film is likely to be deposited on the microwave introducing window rather than on the substrate in forming an a-Si film by using hydrogen gas and silane gas and, consequently, an a-Si film is deposited gradually over the inner surface of the cavity resonator to make the maintenance of discharge and the start of discharge difficult, (v) the A-Si film deposited on the microwave introducing window falls off the microwave introducing window before long and falls on the substrate of the film formed on the substrate to deteriorate the quality of the deposited film, (vi) the interior of the deposition vessel needs frequent cleaning to obviate the deterioration of the deposited film by the film formed on fell off the microwave introducing window, and hence (vii) the rate of operation of the apparatus is low.
The type 2 MW-PCVD apparatus has a further problem adversely affecting the operating efficiency thereof that the end plate 16 of the cavity resonator must be moved under a vacuum to vary the length of the cavity resonator because the microwave introducing window 3 is fastened firmly to the waveguide 4.